1. Origin of the Invention
The invention described herein was made in the performance of work under a NASA Contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958, Public Law 85-568 (72 STAT 435; 43 USC 2457).
2. Background of the Invention
The present invention relates to a process of stabilizing an optical directional coupler switch and to an improved switch produced by such a process, and more particularly to an optical method for reducing cross-talk in ferroelectric crystal waveguide switches.
3. Brief Description of the Prior Art
Integrated optical switches such as titanium-diffused optical waveguides of ferroelectric lithium niobate (LiNbO.sub.3) have led to relatively efficient highspeed electro-optical directional coupler switches and modulators. These devices are capable of being utilized in various fiber optic systems. Their utilization would be more practical if their power level capabilities could be increased.
There are presently available infrared semiconductors lasers that are capable of emitting several milliwatts of power. The optical switches, however, are limited to a few tens of microwatts in the infrared range of 800 to 900 nanometers. Experience has found that cross-talk between the coupled waveguides slowly increases even when the applied field voltage is held constant, at a rate which apparently depends on the optical intensity in the waveguides. This problem has been defined as optical damage in the prior art. Cross-talk drift occurs even at relatively low power levels.
An analysis of this problem was set forth in the January 1980 Journal of Applied Physics, page 90, in the article "Optically-Induced Cross-Talk in LiNbO.sub.3 Switches" by Schmidt et al. The conclusion of the article was that the cross-talk drift at a wavelength of 633 nanometers was believed to occur because of the photorefractive effect of LiNbO.sub.3. The photorefractive effect refers to the optically-induced change of refractive index observed in LiNbO.sub.3 crystals. It was believed that such an index of refraction change is due to the charge transfer of photoexcited carriers from impurities or defects in the crystals to trapping sites. A space charge field is created which causes a change of the refractive index via the electro-optical effect. In the same way, photocarriers, which are excited in the waveguide, drift in the direction of the applied field, thereby reducing the magnitude of the field in the waveguide region. This effect correspondingly increases the optical cross-talk between the waveguides. It has been discovered that increasing the magnitude of the external field after the cross-talk drift has saturated will restore the switch to its original low cross-talk value. The time dependence of the cross-talk level is dependent on the past exposure of the switch since the magnitude of the space charge field, its spatial distribution and its persistence dependent on details of the trapping of carriers in the crystal. The article further described an experiment at 1060 nanometers. While noting that relatively low cross-talk drift was observed, it was unable to account for the amount of refractive index changes that occurred at the longer wavelength. The conclusion to the article suggested that the cross-talk drift problem or optical damage effect could possibly be eliminated by reducing the impurity content of the crystals or by varying the chemical composition or stoichiometry of the waveguides.
The prior art further discloses that optical changes in the index of refraction of dielectric bodies such as lithium niobate have been observed in the presence of visible and ultraviolet electromagnetic radiation such as disclosed in the Chandross et al. U.S. Pat. No. 3,689,264. The invention in this patent was directed at providing an irreversible index of refraction change in polymethacrylate material that was sensitized by introducing peroxide before polymerizing the material at a low temperature with a subsequent exposure to ultraviolet radiation. The Nassenstein et al. U.S. Pat. No. 3,864,130 is cited of general interest to disclose the capability of producing optical waveguides having a gradient refractive index on dielectric material.
The prior art is still seeking a stabilized optical directional coupler switch that is capable of effectively transmitting higher light intensities with a minimum of optical damage or deleterious crosstalk.